Implant and process of modifying an implant surface

ABSTRACT

An implant and a process of modifying an implant surface, which implant is in particular a hip implant, a tooth implant, a bone screw, fixation pin or a fixation nail (pin-fixation), comprising a metallic base body ( 1 ), which implant has a surface modified by a tantalum or niobium, for the formation of a surface modification ( 4 ) where at least the tissue-friendly metal is alloyed with the surface and constitutes a uniform, diffusion-tight outer zone ( 2 ) on the body ( 1 ), which outer zone ( 2 ) has a higher ductility than the metallic base body ( 1 ) in order to obtain a tissue-friendly implant with increased fatigue strength.

[0001] The present invention relates to an implant, in particular a hipimplant, a tooth implant, a bone screw, a fixing pin or a fixing nail(pin-fixation), comprising a metallic base body, which base body has asurface being modified by a material with a tissue-friendly metal chosenamong tantalum and niobium for the formation of a surface modification.

[0002] Implants to be inserted in the human organism must or shouldfulfil several requirements, e.g. that the implant should not releasedangerous substances or cause allergic reactions from the patient.Obviously, this applies in particular to the implants intended forpermanent or lasting placement in the organism, but neitherpin-fixations which are often removed after a few weeks or months shouldrelease dangerous substances or cause allergic reactions. Pin-fixationscould be pins or nails which are inserted through the tissue to a bone,and these pins or. nails are mutually connected to an outer support, theends of e.g. a broken bone being secured in a correct position and theload being transmitted by the pins and support. Pin-fixations may alsobe used alone in a bone in order to hold together bone parts whenhealing and in this case, the pin-fixations will often be left in thebody, thus avoiding further operations which may be traumatic andfurthermore create more cicatricial tissue.

[0003] However, many implants have been produced from materials which donot fulfil this requirement, for example, one material often used is analloy of cobalt, chrome and molybdenum (CoCrMo) (e.g. Vitallium®containing 60% Co, 35% Cr and 5% Mo). It has appeared that the cobaltcontent, among others, in CoCrMo is dissolved and diffused out of theimplant and into the bloodstreams, which is strongly undesirable sinceit may result in poisoning and injuries of organs, including the heart.Another material often used for implants is stainless steel alloyed withnickel which is also an undesired substance as it may cause allergy. Onthe other hand, pure titanium is relatively tissue-friendly and can beused for implants essentially without inconvenience in respect ofallergy or poisoning. However, pure titanium is less suitable forimplants which are exposed to major mechanical forces, such as hipimplants, as implants of pure titanium do not have the same goodstrength properties as the above-mentioned materials and thus, implantsof pure titanium are to a higher degree exposed to fracture thanimplants of the other materials. Therefore, different titanium alloyshave been developed with improved strength properties, but also in thiscase, the alloy substances may cause allergy or poisoning.

[0004] In prior art, several attempts have been made to prevent theimplant from releasing poisonous substances or causing allergicreactions. It is thus well described how the implant can be coated witha tissue-friendly material. WO 99/65537 discloses a metallic implantwith a surface or a surface coating consisting of several layers of e.g.tantalum in layer thickness of 5 μm or more. This implant has thesubstantial disadvantage that the coating may peel off as so far it hasin practice been impossible to make the layers adhere properly to eachother and to the subjacent base body of course, the peeling isparticularly a problem with implants which are driven into a bone withgreat force or which are exposed to major loads when inserted in thebody. In addition to lacking the desired function as protection againstrelease of unwanted substances from the implant, the peeling of thecoating has further the disadvantage that the fixation of the implant isdefective in these areas.

[0005] In order to avoid peeling of a coating, U.S. Pat. No. 4,743,308discloses a process of passivation of a metal alloy, especially aCo—Cr—Mo alloy (Vitallium®) with a coating of tissue-friendly material,such as a noble metal, and exposing the coated surface to a bombardmentby an ion beam which drives the coating into the metal alloy such thatthere is no surface layer which can peel off. However, the ion beambombardment causes that the surface of the implant is not impervious,but almost porous, as it will be spotted with small so-called pin-holesfrom the ion beam bombardment, and thus, there is a risk that theimplant still releases noxious substances.

[0006] Furthermore, certain types of implants are exposed to very heavymechanical loads, e.g. implants for femurs, hip socket and knee, andthese implants must therefore fulfil some severe strength conditions.This is becoming more important as in addition to elderly people needingimplants because of attrition and fracture, e.g. if they suffer fromosteoporosis, there is an increase in the number of young people whoneed such implants due to acute injuries and attrition resulting fromextreme sport activities or the like. It has turned out that knee andhip implants have a durability of 10-15 years which is sufficient inmany cases for elderly people, but not for young people. This is due tothe fact that it is difficult and often impossible to re-operate, sooften an implant cannot be replaced by another as the operation is amajor surgical intervention which may be disabling when repeated.

[0007] U.S. Pat. No. 5,415,704 discloses a surface hardened, metallicimplant where the implant is produced from a metallic alloy added with adissolved, slightly oxidizable or nitridable metal, such as tantalum.The object is to form an oxidized or nitrided surface layer which canboth seal the surface in order to prevent release of poisonoussubstances and harden the surface in order to obtain a high abrasionresistance of the implant. However, this implant does not have adiffusion-tight, uniform surface and the hardened surface might crack ata bending or fluctuating load as the surface will be relative brittleand cannot follow the movements of the base body.

[0008] WO 99/26673 discloses an implant provided with a surface layer.The thickness of the surface layer is chosen such that it is less thanthe critical defect size for the actual material and stresses. Thesurface layer may consist of calcium phosphate or an oxide of titanium,zirconium or tantalum. It is indicated that the thickness of the surfacelayer preferably is less than 5 μm. The object of this invention is toprovide a maximal thickness of a surface layer of a material with lowstrength to prevent the formation of fissures in the layer which mayinitiate cracks in the base body such that the strength of the implantwill be less.

[0009] Finally, DE 19940970 discloses a process for an implant oftitanium or a titanium alloy with a protection layer of TiO₂ and furthera surface coating with calcium. However, this patent was published onlyafter the priority date of the present application.

[0010] One object of the present invention is to provide an implantwhich does not release noxious substances and which has a surface thatcannot peel off.

[0011] A second object of this invention is to provide an implant withan improved mechanical strength.

[0012] The implant according to the invention is characterized in thatat least the tissue-friendly metal is alloyed into the surface andconstitutes a uniform, diffusion-tight outer zone on the base body,which outer zone has a higher ductility than the metallic base body.

[0013] The tissue-friendly metal is alloyed into the surface and thismeans that a metal body in a hardened form is exposed to a process whichalloys another metal to the surface of the body, and that the surfacethus has an alloy zone, the alloyed metal diffusing up to somemicrometers into the body. The application of the alloyed metalcontinues until it forms an outer zone with a uniform, diffusion-tightsurface of essentially pure tissue-friendly metal. The tight outer zoneproceeds gradually to the alloy zone which is structurally anchoredcompletely in the base body. The outer zone has a higher ductility thanthe metallic base body, i.e. that the outer zone has a higherdeformation ability than the base body such that the outer zone can beextended longer than the base body.

[0014] When producing implants, micro and macro cracks are formed on thesurface of the metallic base body, and these cracks cannot be completelyremoved by subsequent treatment, even by polishing with e.g. diamondpaste. Furthermore, the surface of the base body will be provided withgrain boundaries, and both grain boundaries and cracks cause notcheffect during fatigue, thus facilitating the initiation of crack growthfrom the surface of the base body, which may lead to fractures. As theouter zone is uniform and impervious, all cracks and grain boundaries onthe surface of the base body is efficiently sealed. The implant surfaceis free from notch effect, the surface being without grain boundaries,cracks or anything from where a crack may initiate, which entails thatthe fatigue strength is substantially increased. The strength fatigue isof crucial importance to the durability of implants as most implants areexposed to repeated load. A hip implant at ordinary walk will thus beaffected about once per second, which means that for a person being outof bed for about 5 hours a day, the total number of loads in a year willbe more than 6.5 million. Further, the higher ductility of the outerzone in relation to the base body means that this zone follows themovements of the base body and does not peel off.

[0015] An impervious surface without micro porosities is advantageous asbacteria have more difficulties in adhering to an impervious surface,and there is thus less risk of introducing bacteria when inserting theimplant. This means that the healing process is not impeded by bacteriaand the risk of complications is minimized.

[0016] According to a preferred embodiment, the metallic base body is aCo—Cr—Mo alloy which has proved to obtain a particularly high increaseof fatigue strength by surface modification.

[0017] In a preferred embodiment, the base body is modified by a fusedsalt process to a thickness of the outer zone of about 2-14 μm,preferably more than 5 μm and less than 12 μm, and in particular 8-10μm. A thickness of the outer zone of 2 μm may be sufficient by simplegeometrical forms of the base body, but with holes and edges, themodification will result in a thinner outer zone and thus a risk of aporous surface. A thickness of the outer zone of more than 14 μm willentail an increase of the process time and more considerable materialexpenses, and this will thus be unfavourable for economic reasons. Ithas turned out that a thickness of the outer zone of 8-10 μm makes areasonable compromise between certainty of a sufficiently thick outerzone over the entire base body and economy.

[0018] In an alternative embodiment, the base body is modified by a CVDprocess to a thickness of the outer zone of about 10-35 μm, preferablymore than 12 μm and less than 25 μm, and in particular 12-17 μm. Thenecessary thickness of the outer zone is larger by this process, as themodification is effected by columnar growth, which entails a risk ofpin-holes in the surface of thin layers. A thickness of 10 μm will besufficient by simple forms of the base body, but at holes and edges theouter zone will be thinner, and there will thus be a risk of a poroussurface. A thickness of the outer zone of more than 35 μm will entail anincrease of the process time and more important material costs, and thiswill thus be unfavourable for economic reasons. It has turned out that athickness of the outer zone of 8-10 μm makes a reasonable compromisebetween guarantee of a sufficiently thick outer zone over the entirebase body and economy.

[0019] The implant according to the invention may be modified with anysuitable material containing a tissue-friendly metal chosen amongtantalum or niobium, however, in a preferred embodiment, thetissue-friendly metal is tantalum. It has turned out that this materialhas particularly good properties, both as to tissue-friendliness,alloying into the base body, improvement of the fatigue strength of theimplant, and corrosion resistance.

[0020] According to a preferred embodiment, the implant is characterizedin that the tissue-friendly metal is α-tantalum. Tantalum can as anumber of other metals in a solid state occur in more than one type ofcrystal lattice. Tantalum may thus occur as α-tantalum with abody-centred cubic lattice and as β-tantalum with a tetragonal lattice.In this connection, α-tantalum is preferable as it is to a much higherextent than β-tantalum dense and ductile.

[0021] According to a preferred embodiment, the implant is furthercharacterized in that the implant has compressive stresses in thesurface. This contributes to an increase of the fatigue strength of theimplant as a possible tensile load will be counteracted by thesecompressive stresses, and opening of cracks will be counteracted.

[0022] The implant is according to an embodiment characterized in thatthe surface modification also comprises an alloy zone alloyed into thesurface of the base body as in the alloy zone there is a graduallyincreasing concentration of the modification material in the directionthe surface of the outer zone, whereas the concentration of the alloy ofthe base body decreases gradually in the direction of the surface of theouter zone. This alloy zone assures a good adhesive power of the outerzone such that it is to a particularly high degree assured that theouter zone does not peel off or crack.

[0023] According to a preferred embodiment, the implant is characterizedin that the implant has strength properties essentially corresponding tothe strength properties of the bone in which the implant is inserted.This is to be seen in the light of the fact that the modulus ofelasticity in bones is about 21,000 MN/m², whereas e.g. in steel it isabout 10 times as large, i.e. about 210,000 MN/m². The same applies tothe tensile strength in bones which is about 140 MN/m², whereas thetensile strength in typical high-tensile steel is up to 1,500 MN/m².

[0024] Since the outer zone, as mentioned earlier, has a higherductility than the base body, the implant can be made with strengthproperties close to those of the bone, and also yield a little by loadwithout any risk for the outer zone to peel off. In heavy loadedimplants, such as hip implants, in order to counteract bone changes, itis particularly advantageous that the implant has strength propertiesclose to the strength properties of the bones in the body. It is thusassured that no stress concentration occurs to increase the risk offracture or an abnormal growth of the bone, or if the bone implant ismuch stronger 20 than the bone, the risk that the bone shrinks in theareas where the bone is relieved by the implant.

[0025] Furthermore, the surface can be further modified by aningrowth-promoting substance, such as calcium or hydroxyapatite. Thus, agood adhesion of the implant is assured without using cement or anotherfixation material.

[0026] The implant is in particular a fixation pin or a fixation nail(pin-fixation) with a diameter less than 10 mm. It is an advantage withas small dimensions as possible in connection with pin-fixations as theyare to be inserted through the tissue and into the bone and are oftenremoved after some weeks, and the inconveniences in this connection willbe less the smaller dimensions by which these pin-fixations can beproduced. The small outer dimensions are possible as pin-fixations withsurface modification according to the invention have a higher fatiguestrength and can thus be produced with smaller dimensions such that thehole in the tissue and bone will be smaller.

[0027] The invention further relates to a process of modifying animplant surface which process modifies a surface of an implant, inparticular a hip implant, a tooth implant, a bone screw or the like,comprising a metallic base body, which implant is modified by a materialcontaining a tissue-friendly metal chosen among tantalum and niobium.

[0028] The process according to the invention is characterized in thatthe modification comprises a CVD or fused salt process with thetissue-friendly material which is alloyed into the surface of the bodyby the process, whereby the surface has an alloy zone, the alloyedmaterial diffusing up to some micrometers into the body, and the supplyof material continues until an outer zone has been formed with auniform, diffusion-tight surface of essentially pure tissue-friendlymetal, the alloy zone passing gradually into the diffusion-tight outerzone.

[0029] In the following, the invention will explained in more detail bymeans of embodiments and with reference to the accompanying drawing, inwhich

[0030]FIG. 1 schematically shows a sectional view of a surface modifiedbase body,

[0031]FIG. 2 shows a graph indicating the quantity of tantalum which haspenetrated a base body of a Co—Cr—Mo alloy,

[0032]FIG. 3 shows an enlargement at 600 magnification in an electronmicroscope of a base body with surface modification,

[0033]FIG. 4 is a surface modified surface with pinholes,

[0034]FIG. 5 is a surface modified surface without pinholes,

[0035]FIG. 6 shows a graph indicating the quantity of Co in the surfacearea of a tantalum modified base body produced from a Co—Cr—Mo alloy,

[0036]FIG. 7 shows a thin section perpendicular to a fracture zone of asurface modified base body,

[0037]FIG. 8 is a view showing a crack in a base body, and

[0038]FIG. 9 is a graph with curves for the content of tantalum andcobalt in the area around the surface of the base body.

[0039]FIG. 1 schematically shows a base body 1 with a surfacemodification 4 comprising an alloy zone 3 and an outer zone 2, where themodification material in the alloy zone 3 has penetrated the base body1, whereas the outer zone 2 consists of pure modification material.Because of this structure, the surface modification 4 is securelyanchored to the base body 1, thus it is not a coating in a common sense.The thickness of the alloy zone 3 is different depending on the materialfrom which the base body 1 is produced, thus the alloy zone 3 can be inthe order of 0.5 μm-1.5 μm for tantalum on Co—Cr—Mo, whereas it is up to1-10 μmm for tantalum on stainless steel, and 8-10 μm for tantalum on atitanium alloy.

[0040] Other types of tissue-friendly metal than tantalum may be used,however, it has turned out that this material has particularly goodproperties as to tissue-friendliness, alloying into the base body,improvement of the fatigue strength of the implant and resistance tocorrosion. Thus, tantalum is more resistant to corrosion than gold, andat room temperature, has a higher resistance to most acids and bases. Byway of example, a test body of stainless, surgical steel (316L) with adiffusion-tight outer zone of tantalum exposed to an aggressivehydrochloric acid fog was without a trace of breakdown in the tantalumsurface after 30 days, where test bodies without the diffusion tightouter zone corrodes away in a few minutes. In the test, a half-filledcontainer with 40% hydrochloric acid in an aqueous solution was used, ata temperature of 75° C., whereby a gas phase will be formed above thefluid surface—a hydrochloric acid fog which is very aggressive. Testbodies of 6 to 13 mm in diameter were used, and these test bodies wereimmersed halfway into the fluid, such that the rest was in the gasphase.

[0041] The thickness of the outer zone 2 is determined according to thefact that the outer zone 2 has to be uniform and diffusion-tight, andaccording to an embodiment in which a fused salt process is used, thethickness of the outer zone 2 is 8-10 μm. In another embodiment, a CVDprocess is used, which requires a thickness of the outer zone 2 of 15-25μm before it is assured that the outer zone 2 is uniform anddiffusion-tight.

[0042] In the fused salt process, a base body 1 is lowered in a bath ofmelted salt to be covered by a material. Not any metal can withstandbeing lowered in a salt melt as the melt is strongly reactive, and thuse.g. titanium will be dissolved in a moment (fluoride salt melt). Atappropriate control by electric impulses, a uniform diffusion-tightouter zone 2 of tantalum can be obtained with a thickness of the outerzone 2 of about 8-10 μm, and a so-called smooth surface is obtainedwhich is completely even and smooth without grain boundaries. An evenand smooth surface is advantageous as there is a minimum risk ofbacteria on the surface when inserting the implant.

[0043] Another possibility is the CVD process (Chemical VapourDeposition) which is a gas chemical process where a material isvaporized and by means of a carrier gas is brought to a base body wherethe material is deposited on the surface under columnar growth. Becauseof the columnar growth, a thickness twice or three times bigger of theouter zone 2 is required before this is uniform and diffusion-tightwithout pin-holes. On the other hand, the process is environmentallyadvantageous compared to the fused salt process, and this process alsopermits to obtain a good surface modification of inner surfaces, holes,etc. However, this process is carried out at temperatures of about 900°C., which may be a problem in connection with e.g. titanium and itsalloys as a undesired large grain growth may occur at thesetemperatures.

[0044] On the other hand, it is not possible to use e.g. the PVD process(Physical Vapour Deposition), as this process gives an entirely poroussurface with a large number of pin-holes per square centimetre.Furthermore, no outer zones at all can be provided with the necessarythickness, as the material will deposit as a layer on the surface of thebase body without actual alloying, the PVD process taking place attemperatures below 300° C., and therefore the layer will tend to peeloff at a thickness of more than 2-3 μm.

[0045] By the CVD and fused salt process, an alloy of the modificationmaterial is obtained in the surface of the base body 1, the modificationmaterial diffusing a little into the base body 1. This is seen from,among others, FIG. 2 showing a measurement of the content of tantalum atdifferent distances from the surface of a base body produced fromCo—Cr—Mo and surface modified by tantalum. In FIG. 2, a distance of zerorepresents the surface of the base body 1, negative values positions inthe base body 1 and the alloy zone 3, whereas positive values representpositions in the outer zone 2. It can thus be seen that in a depth of100 nm (0.1 μm), there is a weight percentage about 40 of tantalum. Thisindicates that even in a base body made from Co—Cr—Mo having a ratherclosed surface, an alloy of tantalum takes place in the base body 1which assures a complete anchoring of the outer zone 2, and thus thatthe outer zone 2 does not peel off.

[0046]FIG. 3 shows a sectional view of a surface modified base body 1produced from a Co—Cr—Mo alloy modified by tantalum by a fused saltprocess. The outer zone 2 on the base body 1 has in this embodiment athickness of about 15 μm. It has turned out that the thickness of theouter zone 2 when modified by tantalum by the fused salt process doesnot need to be larger than about 10 μm, however, there is nothing toprevent much thicker outer zones 2, e.g. of 50 μm.

[0047] As mentioned, it is essential that the outer zone 2 is uniformand diffusion-tight which may be difficult to obtain, especially by theCVD process where the outer zone 2 is built up by columnar growth asthere is a risk that pin-holes will appear in the surface, this meansthat the outer zone 2 is provided with through-going holes. This is seenin FIG. 4 showing the surface of a surface modified base body. The blackspots are such pin-holes. FIG. 5 shows a surface of a correspondingsurface modified base body, and it can be seen that this surface isimpervious and without pin-holes.

[0048] Since the implant according to the invention has a uniform anddiffusion-tight outer zone, a diffusion barrier is thus provided toassure that unwanted substances in the base body, such as cobalt, do notdiffuse out of the implant. As can be seen from FIG. 6 indicating themeasured quantity of cobalt in the base body, alloy zone and outer zoneof a tantalum modified base body made from a Co—Cr—Mo alloy, themeasured quantity of cobalt reduces in the alloy zone 3 from approx. 65%in the base body 1. Again a distance of zero represents the surface ofthe base body 1, negative values positions in the base body 1 and thealloy zone 3, whereas positive values represent positions in the outerzone 2. In this connection it should be remarked that the figure due tomeasuring technical limitations provides a somewhat misleading picture.In fact, cobalt is only present in the alloy zone where the quantitygradually approaches zero, whereas no cobalt is found in the outer zone.

[0049]FIG. 7 shows a thin section perpendicular to a fracture on acorresponding test piece. It is seen that the outer zone 2 did notloosen or peel off, however, it seems that the outer zone 2 of puretantalum has yielded just at the fracture, which confirms that the outerzone 2 does not peel off and that the outer zone 2 has a higherductility than the base body 1.

[0050]FIG. 8 showing an enlargement of a base body 1 of stainless steelwhich has been surface modified by tantalum, is an example of a crackwhich seems to stop in the outer zone 2 of tantalum, which may be due tothe fact that the outer zone 2 has a higher ductility than the base body1, the concentration of stress at a crack tip being reduced, and thatthere are compressive stresses in the surface of the implant.

[0051] The fact is that X-ray analysis has proved that for a base bodymade from stainless steel and surface modified by tantalum by the fusedsalt process, there are compressive stresses in the surface of the outerzone of about 300 Mpa. These compressive stresses will contribute tocracks not being formed so easily in the surface, the surface beingprestressed and preventing the cracks from opening, as this compressivestress has to be overcome beforehand.

[0052]FIG. 9 is a graph with curves for the content of tantalum andcobalt in the area around the surface of the base body, such that thefigure corresponds to a connection of FIGS. 2 and 6. The figureillustrates that in the alloy zone 3, there is a gradually increasingconcentration of modification material in the direction of the outerzone 2, whereas the concentration of the alloy of the base bodygradually decreases in the direction of the outer zone 2. The outer zone2 consists almost solely of modification material.

[0053] By measurement of the fatigue strength, it has been ascertainedthat by the invention, there is provided an implant which has improvedfatigue strength.

[0054] A base body produced from stainless steel obtains thus animprovement of the fatigue strength of 20% by a surface modification bytantalum, whereas a base body produced by a Co—Cr—Mo alloy obtains animprovement of the fatigue strength of 60%.

[0055] As earlier mentioned, niobium can also be used for the surfacemodification. When niobium is alloyed in the surface, it must, however,be assured that no oxides are formed as this reduces the adherenceconsiderably.

[0056] The invention may also be used for skin lead-throughs andcatheters, i.e. the type of implants which are not anchored in a bone.Such lead-throughs are used by ostomy and dialysis for transportation ofliquids and substances out of and into the body, but may also be usedfor electric lines, e.g. in the case where a pacemaker is positionedoutside the body. In some cases, the lead-through is of permanentcharacter, and in other cases of temporary character. The lead-throughconsists of an L, T or I-shaped metal tube, where the surface of thetube is modified according to the invention.

1. An implant, in particular a hip implant, a tooth implant, a bonescrew, a fixing pin or a fixing nail (pin-fixation), comprising ametallic base body (1), which base body has a surface being modified bya material with a tissue-friendly metal chosen among tantalum andniobium for the formation of a surface modification (4), characterizedin that at least the tissue-friendly metal is alloyed into the surfaceand constitutes a uniform, diffusion-tight outer zone (2) on the basebody (1), which outer zone (2) has a higher ductility than the metallicbase body (1).
 2. An implant according to claim 1, characterized in thatthe metallic base body (1) is a Co—Cr—Mo alloy.
 3. An implant accordingto claim 1 or 2, characterized in that the base body is modified by afused salt process to a thickness of the outer zone (2) of about 2-14μm, preferably more than 5 μm and less than 12 μm, and in particular8-10 μm.
 4. An implant according to claim 1 or 2, characterized in thatthe base body (1) is modified by a CVD process to a thickness of theouter zone (2) of about 10-35 μm, preferably more than 12 μm and lessthan 25 μm, in particular 12-17 μm.
 5. An implant according to any ofthe preceding claims, characterized in that the tissue-friendly metal istantalum.
 6. An implant according to any of the preceding claims,characterized in that the tissue-friendly metal is α-tantalum.
 7. Animplant according to any of the preceding claims, characterized in thatthe implant has compressive stresses in the surface.
 8. An implantaccording to any of the preceding claims, characterized in that thesurface modification (4) also comprises an alloy zone (3) alloyed intothe surface of the base body (1) as in the alloy zone (3) there is agradually increasing concentration of the modification material in thedirection of the surface of the outer zone (2), whereas theconcentration of the alloy of the base body decreases gradually in thedirection of the surface of the outer zone (2).
 9. An implant accordingto any of the preceding claims, characterized in that the implant hasstrength properties essentially corresponding to the strength propertiesof the bone in which the implant is inserted.
 10. An implant accordingto any of the preceding claims, characterized in that the implant is afixation pin or a fixation nail (pin-fixation) with a diameter less than10 mm.
 11. An implant according to any of the claims 1-10, characterizedin that the surface of the implant can be further modified by aningrowth-promoting substance, such as calcium or hydroxyapatite.
 12. Aprocess of modifying an implant surface which modifies a surface of animplant, in particular a hip implant, a tooth implant, a bone screw orthe like, comprising a metallic base body (1), which implant is modifiedby a material containing a tissue-friendly metal chosen among tantalumand niobium, characterized in that the modification comprises a CVD orfused salt process with the tissue-friendly material which is alloyedinto the surface of the body (1) by the process, whereby the surface hasan alloy zone (3), the alloyed material diffusing up to some micrometersinto the body (1), and the supply of material continues until an outerzone (2) has been formed with a uniform, diffusion-tight surface ofessentially pure tissue-friendly metal, the alloy zone (3) passinggradually into the diffusion-tight outer zone (2).